Technical Field
The technology relates to circuits, systems, and methods for measuring and compensating a DC-transfer characteristic of analog-to-digital converters.
Discussion of the Related Art
Analog-to-digital converters (ADCs) are widely used in various electronic apparatus and systems such as mobile phones, audio equipment, image-capture devices, video equipment, wireline communications systems, sensors and measurement equipment, and radar systems, amongst others. A typical ADC is an electronic circuit configured to receive an analog signal, which typically is a time-varying signal, repeatedly sample the analog signal at discrete time intervals, and output a digital signal (e.g., a bit sequence or digital word) for each sampled time interval that is representative of a value of the analog signal during the sampling interval. Because the output of an ADC is a bit sequence, the analog signal is discretized into a number M=2N of integer values. The number N is referred to as the bit resolution of the ADC. For example, if an ADC is an 8-bit device, then an input signal can be discretized into 2N=256 values (e.g., 0, 1, 2, 3 . . . 255). In some cases, the discretized values may range from negative to positive values (−64, −63, . . . −1, 0, 1, . . . 62, 63). For an ideal ADC, the output bit value will be linearly proportional to the sampled analog signal value over a full input voltage range that is accepted by the ADC.
There are several types of conventional ADCs, and they may be divided into two groups: ADCs with single-bit quantization (e.g., a comparator) and ADCs with multi-bit quantization (e.g., N-bit ADCs). Multibit quantizing ADCs may include voltage-controlled-oscillator (VCO)-based ADCs and sigma-delta-modulator-based ADCs among others. Multibit quantizing ADCs typically exhibit nonlinear quantization characteristics when converting analog signals to digital signals. These nonlinear characteristics might result in gain error, offset error, quantization noise, flicker noise, and/or other signal-distortion errors for a converted analog signal. As a result, conventional ADCs typically do not ideally convert analog signals to digital signals, so that additional measures may need to be taken to correct the converted signals and increase the accuracy of an ADC.
Some approaches for reducing nonlinear quantization errors have been described in the literature. For example, dynamic element matching has been described and allows converting signal distortion of an ADC into noise. Alternatively, nonlinear quantization characteristic can be measured and stored in digital or analog memories. These stored values can then be used later to perform a correction during the analog-to-digital conversion. This approach is described, e.g., in Larson, L. E., Cataltepe, T., and Temes, G. C., “Multibit Oversampled ΣΔ A/D Converter with Digital Error Correction,” Electronics Letters, 24: 1051-1052, August 1988.